The development of dielectric resonators for telecommunications has experienced rapid growth in the past decade. Owing to the high dielectric constants, quality factors, and near-zero temperature coefficients of dielectric resonators, microwave-sensing receivers have been dramatically miniaturized. A variety of applications utilizing these relatively low-cost ceramics have been developed for various applications such as personal communication systems, global positioning systems, and personal digital cellular systems.
Several microwave dielectric resonator materials have been developed in recent years. Microwave signals are generally regarded as having signal frequencies in the range from 0.4 to 200 GHz. Dielectric materials are especially useful for resonator and other microwave wireless communications over the 0.5-20 GHz range. A highly attractive material is (Zr.sub.08 Sn.sub.0.2) TiO.sub.4 which has a dielectric constant of 38, a quality factor of 8400 at 7 GHz, and a temperature coefficient of -0.1 ppm/.degree. C. However, the drawback of using this ceramic is the difficulty in machining the sintered pellets to desired dimensions.
Another popular microwave ceramic adopts a complex perovskite structure in the form of A(Bi.sub.1/3 E.sub.2/3 O.sub.3), where A is selected from the group consisting of Ba and Sr; B is selected from the group consisting of Ni, Ca, Zn, Mg, Co and Zr; and E is selected from the group consisting of Ta and Nb. Although Ba(Zn,Ta)O.sub.3 has a high quality factor (10000 at 7 GHz), dielectric constant (29), and temperature coefficient (1 ppm/.degree. C.), the sintering process requires higher temperature (1500-1600.degree. C.) and prolonged sintering times for ordering Zn and Ta cations. An unavoidable disadvantage of using this microwave ceramic in designing a lightweight device is its higher density (7.7 g/cm.sup.3) relative to other material candidates.
Ba.sub.2 Ti.sub.9 O.sub.20 has also received attention for its good microwave properties, quality factor (10,000 at 4 GHz), dielectric constant (39.8), and temperature coefficient (2 ppm/.degree. C.). Titanium ions are located at octahedral sites. This pseudo-hexagonal arrangement has a nine layer stacking sequence with a primitive triclinic cell. Eight barium ions reside in a triclinic cell; four of them are 12-coordinated by oxygen while the other four have a vacancy present in the adjacent barium ions. It has been suggested that this Ba-vacancy-Ba sequence may facilitate better dielectric properties than the other barium polytitanates. However, it has been pointed out that Ba.sub.6 Ti.sub.17 O.sub.40 and Ba.sub.4 Ti.sub.13 O.sub.30 do not have superior dielectric properties, though they have more barium vacancies per unit cell. Further investigation of microwave properties by analyzing far-infrared reflection spectra using the dielectric dispersion equation has been hampered by the complex crystal structure of Ba.sub.2 Ti.sub.9 O.sub.20. (Zr.sub.0.8 Sn.sub.0.2)TiO.sub.4 and A(B.sub.1/3 E.sub.2/3)O.sub.3 (perovskite) have simpler structures and thus, theoretical predictions have been made for their dielectric properties at microwave frequencies.
Ba.sub.2 Ti.sub.9 O.sub.20 has been a difficult phase to fabricate without batch additives which form a solid solution. Tin ion dopants have been used in the form of SnO.sub.2 and BaSnO.sub.3 batch additives in several studies to stabilize Ba.sub.2 Ti.sub.9 O.sub.20, and their microwave properties have been evaluated. The substitution of Sn.sup.+4 for Ti.sup.+4 lowered the temperature coefficient without significantly degrading the dielectric constant. U.S. Pat. No. 4,563,661 to O'Bryan, et al., involves an apparatus for processing microwave electrical energy formed from a barium-titanium compound having nominal formula Ba.sub.2 Ti.sub.9 O.sub.20 doped with Sn.
Nd.sub.2 O.sub.3 additions have been used to stabilize Ba.sub.2 Ti.sub.9 O.sub.20. Nd.sub.2 O.sub.3 additions increased the dielectric constant and temperature coefficient but lowered the quality factor. It has been suggested that the quality factor could be improved with the addition of Mn acting as an oxidizing agent. Addition of up to 5 mol % Sr.sup.+2 reportedly stabilized Ba.sub.2 Ti.sub.9 O.sub.20.
Although it has been noted that zirconium substitution also has stabilizing effects, the microwave properties of Zr.sup.4+ -doped Ba.sub.2 Ti.sub.9 O.sub.20 have not been thoroughly evaluated.
Accordingly, there is a need in the industry to provide for an alternative doping mechanism for Ba.sub.2 Ti.sub.9 O.sub.20 that enhances the use of Ba.sub.2 Ti.sub.9 O.sub.20 in microwave wireless communications applications.